Cat redirection - Bash

Read a million replies to this question but still don’t get it! lol Please help.

What is the difference between:

$ cat filename.txt

&

$ cat < filename.txt

The output is the same.

Existing answers say something along the lines of using < means that cat doesn’t open the file but the file is redirected and the shell opens it as standard input. Something like that.

What does this mean though practically? What does Cat opening the file vs shell opening the file really mean under the hood?

Thank you

It’s not a short read but this SO answer seems to cover what you’re after with a bit more detail under the hood-

this is actually really well laid out! it goes above my beginner skill, but thank you for this! it will probably make more sense later on :+1: